The present invention relates to an apparatus for the color synchronization of reproduced video signals, and more particularly, to an apparatus for effecting the external synchronization of color video signals reproduced from video disk players.
FIG. 1 shows a conventional color synchronizing system, wherein a reproduced video input signal passes to a variable delay line 1 and thence to an external circuit as a time-base controlled video output. The reproduced video signal is also applied to one of the two inputs of a PD (phase detector) 21, whose other input is supplied with an output from a VCO (voltage-controlled oscillator) 24. The output of PD 21 is sampled in a S/H (sample and hold) circuit 22 for each horizontal scanning period (1H), and the hold output of the S/H circuit 22 controls the variable delay line 1 while it is used as a signal for controlling VCO 24 through a LPF (low-pass filter) 23.
The reproduced video input signal is also fed into a sync separator 3 and, after a horizontal sync signal is separated therefrom, it is supplied to one input of a PD 4. The separated horizontal sync signal is fed into a burst gate generator 5, which generates sampling signals to control the sampling at the S/H circuit 22 as long as the color burst is on. The other input of PD 4 is supplied with a selective output from a switch 6 that selects either external or internal synchronization. In an external synchronization mode (when the contact indicated by the solid circle in switch 6 is made), an external sync signal is selected, and in an internal synchronization mode, an internal sync signal from an internal sync generator 7 is selected.
The circuit shown in FIG. 1 also includes PD 8 which receives at its two inputs the output of the variable delay line 1 and an external subcarrier. The output of PD 8 is fed through a S/H circuit 9 and an equalizer 10 to be received at an external synchronization input (or the contact indicated by the solid circle) in switch 11 that selects either external or internal synchronization. The operation of the S/H circuit 9 is controlled by sampling signals generated at the burst gate generator 5. The internal synchronization input of the switch 11 is grounded.
The output of the switch 11 and the PD 4 are summed in an adder 12, and the summed signal is fed through equalizers 13 and 14 which provide servo signals for a tangential mirror (not shown) and a spindle motor (not shown).
The reproduced signal entering the circuit shown above contains jitters, and in order to absorb them, a 3.59 MHz continuous wave which is synchronous with the color burst for the reproduced video signal is generated in the PLL circuit 2 so that the amount of delay in the variable delay line 1 is controlled by using the phase error caused in the PLL circuit 2 as a time base error signal. This enables the proportional control of the reproduced video signal, and the input/output characteristics of the phase in the variable delay line 1 are represented by: EQU G'(s)/(1+G'(s)) (1)
wherein G'(s) is the open loop gain of the PLL loop 2.
FIG. 2 shows the schematic functional block diagram of FIG. 1 in an external synchronizaiton mode, and includes a spindle motor 15, a pickup 16 and a modulation circuit 17. The PD 8, adder 12, spindle motor 15, pickup 16, modulator 17 and variable delay line 1 form a color loop that achieves color phase matching between the reproduced signal and the external subcarrier. If the closed-loop transfer function of this color loop excluding the variable delay line 1 is written as G(s), the open-loop characteristic of the whole system is calculated as follows from formula (1): EQU G(s).multidot.G'(s)/(1+G'(s)) (2)
This indicates the inclusion of a small proportional-control loop within a large color loop.
Since the proportional-control loop included in the color loop for absorbing jitters has a PLL circuit, the system shown in FIGS. 1 and 2 includes a circuit having the transfer function G'(s)/(1+G'(s)) which represents the closed-loop characteristics of the PLL. The PLL is usually driven in a relatively low loop frequency band (f.sub.c =10 Hz) so that it will not be responsive to higher frequencies. This low loop frequency band is necessary for extracting a time-base error value from the low-frequency component.
In internal synchronization mode, any jitter and color fluctation can be eliminated in both low and high frequency ranges. However, in external synchronizaion mode, the color loop has the closed-loop transfer function represented by formula (2) and since it acts as if it contained a low-pass filter, it is unable to achieve a very large loop gain. Because of this insufficiency of loop gain, the system shown in FIGS. 1 and 2 exhibits a stationary phase error too great to ensure phase matching between the external subcarrier and the output video signal.
Instead of supplying the output of the variable delay line 1 to one input of PD 8 in FIG. 1 or 2, a portion of the input to the variable delay line 1 may be fed to that input of PD 8; this eliminates the proportional-control circuit loop from the color loop, thereby increasing the gain of the color loop. However, the greatest problem with this method is that if the amount of absolute delay in the variable delay line changes by temperatures and other factors, the color phase of the output video signal may become offset from the external subcarrier (the variable delay line usually employs a variable-capacitance diode and is subject to considerable variations in the amount of absolute delay due to temperature changes).
Another problem arises with respect to external synchronization when the reproduced video input signal undergoing external synchronizaton is a PAL color video signal.
The PAL system color video-signal E.sub.P is expressed by the following equation: EQU E.sub.p =Y+(B-Y) sin .omega..sub.s t.+-.(R-Y) cos .omega..sub.s t (3)
where
Y denotes a luminance signal, PA1 B and R denote blue and red signals, and PA1 .omega.hd s denotes the angular frequency of a color subcarrier wave which is about 4.43 MHz. PA1 (1) Synchronizing both H (horizontal) and V (vertical) synchronizing signals with H and V external synchronizing signals; PA1 (2) Synchronizing color burst signals with an external reference subcarrier; and PA1 (3) Precisely holding the inverse condition of PAL phase (which means the phase of both the color burst signals and the (R-Y) cos .omega..sub.s t signal.)
In Eq. (3), the signal indicates phase-inversion per horizontal scanning line. The (R-Y) component of a color carrier signal issues a subcarrier signal .+-.cos .omega..sub.s t having a phase difference of 180 deg. between scanning lines after AM-modulation. Therefore, on a TV receiver side, a burst signal has to have additional information for phase inversion, at each line, of the subcarrier for remodulation of the (R-Y) component. Accordingly, as understood from the vector diagram shown in FIG. 3, a color burst signal is changed over at each line so as to have a phase of .+-.135 deg. about the (B-Y) axis and is issued as a phase-inversion signal sin (.omega..sub.s t.+-.135 deg.).
The following steps must be carried out in the case of the external synchronization of such a PAL system color video signal for image-synthesization:
In particular, with respect to the PAL phase in (3) above, color tones sometimes may be precisely reversed with respect to the (B-Y) axis upon image synthesization or the like. In a VDP (video-disc player) for example, when scanning and track jumping are performed in special reproduction, the condition of PAL phase inversion changes at a certain point in time (e.g. during track jumping.)